1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and in particular, relates to a structure to apply a downward force to a feeding roller which feeds a sheet stacked on a sheet stack tray.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer and a copying machine is provided with a sheet feeding apparatus including a sheet feeding cassette being a sheet storage portion in which sheets are stacked and a feeding portion which feeds sheets stacked in the sheet feeding cassette as separating one by one. An example of such a sheet feeding apparatus includes a feeding roller which feeds sheets and a separation roller which separates sheets as being abutted to the feeding roller. Further, in the sheet feeding cassette, a sheet stack tray on which sheets are stacked is arranged movably in an up-and-down direction and a sheet feeding force is generated by pressing the sheets to the feeding roller as applying a force to the sheet stack tray with a spring.
Then, the feeding roller is rotated as being pressed to an uppermost sheet stacked on the sheet stack tray to feed a sheet, so that the uppermost sheet is to be fed. Subsequently, the sheet is separated one by one while the fed uppermost sheet passes through a nip of the feeding roller and a separation roller to which a torque limiter to be pressed to the feeding roller is coaxially arranged. Here, the sheet separated one by one is fed to a conveying path toward an image forming portion (see Japanese Patent Laid-Open No. 2009-007086).
By the way, recently, it has been desired to increase an amount (the number) of sheets which can be stored in a sheet feeding cassette to reduce sheet replenishment frequency. However, with a structure to push up a sheet stack tray with a spring toward a sheet feeding roller, following problems occur. Here, large-sized sheets and small-sized sheets are different in weight. In a case that the number of sheets to be stacked on the sheet stack tray is increased, the weight difference between large-sized sheets and small-sized sheets becomes large at the time of being fully-stacked.
In a case that the spring is set for sheet feeding pressure (pressure of the sheet feeding roller abutting to a sheet upper face) of small-sized sheets, sheet non-feeding occurs as the sheet feeding pressure of the sheet feeding roller becomes smaller owing to the fact that sheet weight becomes larger when large-sized sheets are to be fed. In a case that the spring force is set large as corresponding to large-sized sheets, double-feeding occurs as the sheet feeding pressure becomes excessively large as a result of excessively large pressing force when small-sized sheets are stored.
Further, variation of the sheet feeding pressure is largely influenced by a density and a basis weight of sheets as well as a sheet size. For example, a density of certain sheet type could be twice or more than that of a different type. Further, there is a case that densities of sheets having the same size could differ on the order of 30%. Variation of the sheet feeding pressure owing to the density difference is large with increase of the number of stacked sheets.
In the conventional sheet feeding apparatus described above, it is possible to adjust sheet feeding pressure in accordance with sheet size. However, the sheet feeding pressure cannot be adjusted in accordance with density and basis weight of sheets. Accordingly, as the number of sheet types which can be supported by an image forming apparatus is increased, it becomes more difficult to satisfy both sheet feeding performance and enlarging of sheet stacking capacity.
To address the above issues, the present invention provides a sheet feeding apparatus and an image forming apparatus capable of stably performing sheet feeding even in a case that sheet stacking capacity is enlarged.